Rubber goods such as tire treads often are made from elastomeric compositions that contain one or more reinforcing materials such as, for example, particulate carbon black and silica; see, e.g., The Vanderbilt Rubber Handbook, 13th ed. (1990), pp. 603-04.
Good traction and resistance to abrasion are primary considerations for tire treads; however, motor vehicle fuel efficiency concerns argue for a minimization in their rolling resistance, which correlates with a reduction in hysteresis and heat build-up during operation of the tire. These considerations are, to a great extent, competing and somewhat contradictory: treads made from compositions designed to provide good road traction usually exhibit increased rolling resistance and vice versa.
Filler(s), polymer(s), and additives typically are chosen so as to provide an acceptable compromise or balance of the desired properties. Ensuring that reinforcing filler(s) are well dispersed throughout the elastomeric material(s) both enhances processability and acts to improve physical properties. Dispersion of filler particles can be improved by increasing their interaction with the elastomer(s) and/or decreasing their interaction with each other. Examples of efforts of this type include high temperature mixing in the presence of selectively reactive promoters, surface oxidation of compounding materials, and surface grafting.
The section of a polymer chain from the site of the last crosslink to an end of the polymer chain is a major source of hysteretic losses; this free end is not tied to the macro-molecular network and thus cannot be involved in an efficient elastic recovery process and, as a result, energy transmitted to this section of the polymer (and vulcanizate in which such polymer is incorporated) is lost as heat. Ensuring that these polymer chain ends are tied to, or otherwise interact well with, reinforcing particulate fillers, is important to many vulcanizate physical properties such as, for example, reduced hysteresis.
Chemically modifying the polymer, typically at a terminus thereof, is one of the most effective ways of increasing interactivity of fillers and polymers. Terminal chemical modification often occurs by reaction of a living polymer with a functional terminating agent. Some of the numerous examples of this approach include U.S. Pat. Nos. 3,109,871, 4,647,625, 4,677,153, 5,109,907, 6,977,281, etc., as well as references cited therein and later publications citing these patents. Some of the most commonly employed synthetic elastomeric materials used in the manufacture of vulcanizates such as tire components include high-cis polybutadiene, often made by processes employing catalysts, and substantially random styrene/butadiene interpolymers, often made by processes employing anionic initiators. Post-polymerization terminal functionalization that can be performed with anionically initiated polymers often cannot be performed on coordination catalyzed polymers and, to a lesser extent, vice versa.
Terminal modification also can be provided by means of a functional initiator, in isolation or in combination with functional termination. Functional initiators typically are organolithium compounds that additionally include functionality, typically functionality that includes a nitrogen atom, capable of interacting with one or more types of particulate filler materials. Many of these functional initiators have relatively poor solubility in hydrocarbon solvents of the type commonly used in anionic (living) polymerizations, and many do not maintain propagation of living ends as well as more common alkyllithium initiators such as n-butyllithium. Both of these characteristics can negatively impact polymerization rate and efficiency.
This degradation in the ability to propagate living ends can be countered somewhat by conducting polymerizations at lower temperatures, i.e., less than 100° C., often less than 90° C. or even 80° C. Such temperature control is not always possible or practical, however, particularly when a continuous or semi-continuous polymerization process is desired.
Functional initiators that retain good initiation performance in anionic polymerizations conducted at elevated temperatures remain desirable.